In the manufacture of quartz glass bodies according to the so-called OVD (outside vapor deposition) method, SiO2 particles are formed by flame hydrolysis by using one or more deposition burners and are deposited on the outer or circumferential surface of a carrier rotating about its longitudinal axis, resulting in the formation of a cylindrical blank of transparent or porous quartz glass. The deposition burners used for this purpose consist of quartz glass or of metal.
To accelerate the deposition process, several deposition burners are frequently used that are reversingly moved along the blank in a joint row of burners, each deposition burner only sweeping over part of the length of the blank. Especially with blanks consisting of porous quartz glass (so-called “soot bodies”), particular attention is paid that all of the deposition burners show deposition characteristics that are as uniform as possible because, otherwise, this would result in a locally irregular soot density or soot quantity, which leads to axial geometric variations and inhomogeneities of the material and to disturbances particularly in the contact area between neighboring deposition zones. Therefore, a great number of measures have been described for ensuring uniform and reproducibly adjustable deposition characteristics of the deposition burners.
DE 100 18 857 A1, for instance, suggests a deposition burner of quartz glass which consists of four concentrically arranged quartz glass tubes forming a central nozzle surrounded by a total of three annular gap nozzles. The central nozzle is fed with SiCl4, and the outer annular gap nozzles with fuel gases in the form of hydrogen and oxygen. The central nozzle and the outer portion have provided thereinbetween a separation gas nozzle through which an oxygen flow is passed that shields the SiCl4 stream against the fuel gas streams. To exchange burners of a row of burners without any great matching and adjusting efforts, attention is paid in each of the deposition burners to an exact dimensional accuracy of the annular gaps and each of the deposition burners is equipped with a separate positioning unit.
WO 82/03345 discloses a different deposition burner of quartz glass for the fabrication of preforms for optical fibers according to the VAD method. This burner is composed of a multitude of quartz glass tubes arranged in coaxial fashion relative to one another so that a plurality of annular gap nozzles surround a central middle nozzle. At the distal end facing away from the burner mouth the quartz glass tubes are offset in stepped fashion and are fixed in a metallic holder. Inside the holder annular chambers are formed that are fluidically separated from one another and sealed outwards and communicate with the middle nozzle and the annular gap nozzles. Process media are supplied via these chambers to the nozzles.
A similar coaxial burner for the fabrication of preforms according to the VAD method is also known from U.S. Pat. No. 4,474,593 A. This burner has the special feature that the inner nozzle is supported to be displaceable in the direction of the longitudinal axis, so that its position can be varied relative to the remaining annular nozzles during the deposition process. The individual annular nozzles terminate in the area of the burner mouth either in a joint plane or in stepped planes.
JP-8-059260 A discloses a method for making a generic burner. The publication suggests the manufacture of a quartz glass body by elongating a coaxial assembly of quartz glass tubes. The resulting product is usable as an insulating tube for a thermocouple, as a protecting tube for a sensor or a heater, as a gas supply tube, as a member for instruments, as a member for holding wafers in the manufacture of semiconductors, or as a burner head.
In the known deposition burners of quartz glass, contamination of the SiO2 soot body by abrasive wear from the material of the nozzles need not be feared. On the other hand, precise fabrication and orientation of the individual quartz glass tubes is complicated, especially when traditional glass-blowing methods have to be employed. Likewise, it is troublesome to compensate for possible differences in the burner characteristics by positioning the deposition burners individually.
As an alternative, deposition burners are used consisting of special steel or aluminum. Such a metal burner is e.g. known from U.S. Pat. No. 5,599,371 A. The deposition burner is composed of a multitude of metallic nozzle parts which are interconnected by means of screws and form gas chambers fluidically separated from each other.
Since in deposition burners of metal the individual nozzle parts can be manufactured in a precise and reproducible manner by way of the known mechanical machining methods such as drilling, punching, milling, or the like, complex constructions with narrow manufacturing tolerances can also be realized in a comparatively easy way. For series manufacture specifically adapted tools are normally used, the production of which presents a considerable cost factor. A complex burner construction requires the provision of a great number of such tools that, however, as has been shown, are subject to rapid wear due to the hardness of the metallic material, whereby the manufacturing precision is decreasing. Moreover, contamination of the quartz glass by constituents of the high temperature-resistant metallic material must be expected in metal burners.